1. Field of the Invention
The invention relates to a process of correction of measuring signals provided by a radiation detector of the bar detector type. It also relates to a correction device in which this process is used.
2. Discussion of Background
Bar-type radiation detectors can be used to pick up various types of radiation, for example, light, x radiation, alpha-radiation, beta-radiation, gamma-radiation, neutrons, etc. . .
These detectors generally consist of elementary sensors placed behind one another most often along a single line. These elementary sensors are frequently of the photosensitive type, and in the case of radiations other than the light, a converter of energy into light energy and placed between these photodetectors and the radiation source, the converter or scintillator element being a substance suitable for the nature of the radiation.
One of the important applications of the bar detectors or linear detectors is the scanning imagery. The scanning imagery with such detectors is used in particular in the following cases:
for objects in movement of translation, for example, a production line conveyor belt;
in the case where very high dynamics of detection is essential: the linear detectors or bar detectors are then preferable to the matrix- or surface-type detectors in which the photosensitive surfaces are small in particular to reserve a surface required by the presence of many conductors placed in lines and in columns;
in the case of objects of large dimension, a case in which it is possible to use linear detectors which are modular and can be joined;
in the case of x rays, in industrial engineering as well as in radiodiagnostics: scanning radiology is particularly advantageous because it makes it possible to obtain an excellent rejection of the diffused radiation.
By taking, for example, the bar-type detectors or the linear detectors used for the scanning imagery by x rays, for example for the recognition of an object in movement on conveyor belts such as in the luggage inspection installations usually encountered in airports:
the linear detector is often stationary, and it consists of multiple elementary sensors, placed along the same line which is approximately perpendicular to the direction of movement of the objects;
a source of x rays is placed so that the conveyor belt passes between this source and the linear detector;
the x radiation emitted by the source passes through collimation means which impart to it the shape of a fan-shaped beam whose plane is placed along the length of the linear detector;
frequently, the scintillator consists at the level of each elementary sensor, i.e. a layer of scintillating substance is deposited on each elementary sensor.
Thus, the relative movement between, on the one hand, the object, and, on the other hand, the unit formed by source X and the linear detector makes it possible to produce an image of the object which corresponds to the absorption in transmission of the x radiation by the object. Along the linear detector, the definition of this image depends on the number of elementary sensors, while concurrently to the movement of the object, the definition of the image is given by the number of measurements performed by each elementary sensor. The same elementary sensor is used for the acquisition of a large number of image points, all the values of the image achieved with the same elementary sensor during the analysis of the object forming the same line or column of the image of this object.
A free zone of the conveyor belt, located in front of the first object to be examined, is often used to develop one or more correction coefficients, in particular for a correction of the gain and for a correction of the dark signal of each elementary sensor.
Actually, each elementary sensor has a characteristic gain and delivers a characteristic dark signal due in particular to electric leakage currents. Also, to make a good-quality image, it is necessary to correct each measuring value corresponding to a point of the image, with corrective terms characteristic of the elementary sensor having delivered the measuring value.
For a surface (matrix) sensor, a precision of 1 to 3% in corrective terms is generally sufficient, because the effects of correction errors do not have any correlation between points of the image.
In the case of a linear detector, the same elementary sensor is used for the acquisition of a large number of points which form, for example, a line of the image. An inaccuracy in the acquisition of corrective terms characteristic of each elementary sensor affects an entire line of the image, so that the errors in the values of the image points are correlated on the same line. The eye is very sensitive to such correlated errors, which makes it necessary to search for inaccuracies in the correction coefficients lower than 1% and often close to 10.sup.-3.
For each elementary sensor, the corrective terms of the dark current and of the gain are obtained by processing a large number of signals delivered by each elementary sensor, to minimize the effects of the temporary noise of the elementary sensor or the radiation flux. These signals provided by each sensor can be processed according to a true average method or else according to a so-called moving or recursive average method.
FIG. 1 is a diagram of functional blocks of an electronic unit, which makes it possible to illustrate a process of the prior art to develop, and to store, corrective terms intended to correct the values of the measuring signals delivered by each elementary sensor of a linear radiation detector.
In the example which is described below, the dark current values of each of the sensors of a linear detector or bar detector DB are determined by a moving average method.
Linear detector DB is of the type, for example, used in airports for the inspection of luggage. Linear detector DB comprises multiple elementary sensors C1, C2, . . . , Cn, placed on a single line, and which are each linked to an electronic switching device or multiplexer 2 to which each of the sensors delivers a measuring signal S1, S2, . . . , Sn. These signals S1 to Sn are intended to be applied, one after the other, to an analog-to-digital converter 3 to which multiplexer 2 is linked; for this purpose, multiplexer 2 selects one of signals S1 to Sn and delivers through its output a selected measuring signal SMS which is applied to analog-to-digital converter 3. Selected signal SMS is in an analog form, and it is converted into digital data by converter 3; this digital data is available at the output of analog-to-digital converter 3 on parallel connecting lines or bus 4. Bus 4 is linked, on the one hand, to a subtracting device 5, and, on the other hand, to an adding device 6, to which it applies the digital data which corresponds to signal SMS selected from signals S1 to Sn delivered by elementary sensors C1 to Cn.
The data transported by bus 4 represents a rough value, not corrected by the corrective term, and subtractor 5 delivers corrected data DC which is applied to a main acquisition device DPA. This acquisition device DPA has the function, in a conventional manner, of processing and storing corrected data DC, to construct an image of an object (not shown) formed with an x radiation, which, after having passed through this object, has been converted into light so as to be picked up by the photosensitive cells of each of elementary sensors C1 to Cn.
For a good understanding of the following, it is useful to specify some details of operation of the x-ray luggage inspection equipment. The operation of this equipment can be broken down into three phases:
in a first phase, measuring signals S1 to Sn are acquired from each of sensors C1 to Cn in the absence of x radiation; signal S1 to Sn which is then delivered by each of the elementary sensors corresponds to the dark signal or dark current characteristic of each of these sensors. For each of these elementary sensors, the value of the dark current is determined after a large number of acquisitions of signals S1 to Sn. These signals S1 to Sn are converted into digital data by same converting device 3, also these acquisitions are made sequentially, i.e. that first measuring signal S1 is processed first of all, then second signal S2, and so on until signal Sn, which is the last one of an acquisition cycle; and first signal S1 is then taken up again during the next acquisitions cycle. At the end of the acquisition cycles in this first phase, the value of the corrective term for the dark current is set and can be used to be subtracted at the level of subtractor 5 of the rough data delivered by converter 3.
the second phase of operation can be used in the case of a gain correction. For this purpose, a large number of acquisition cycles of signals S1 to Sn delivered by sensors C1 to Cn is made in the presence of an x radiation, but without the radiographed object being placed between linear detector DB and the x-ray source. This corresponds to an illumination of calibration. It should be noted that current I or signal delivered by an elementary sensor, elementary sensor of photosensitive type, for example, can be defined by the following relation: I=I.sub.o +k. E, or I.sub.o, is the dark current, i.e., the current in the absence of illumination (without x rays); k is the coefficient or gain characteristic of each elementary sensor, and E is the illumination. Illumination E is a known constant, and the value of dark current I.sub.o having been determined in the first phase of operation, the value of gain k can be deduced for each elementary sensor and this value can be refined by a large number of acquisitions;
finally, the third phase is the measuring phase in which the radiographed object moves between the source and the linear detector. In this phase, signals S1 to Sn are corrected firstly by the corrective terms of dark current, then next by the corrective terms of gain before being transmitted to an acquisition device used to form the image of the radiographed object.
Unit 1 of the prior art shown by way of example in FIG. 1 makes it possible to define the value of dark current I.sub.o in a first phase such as mentioned above, in which the operation is performed without x radiation.
Linear detector DB and multiplexer 2 are linked to a second connecting network or bus 7 to which are also connected an address counter 8 and a read-write memory (RAM) 10; address counter 8 is synchronized by a clock 9, and it has the function in particular of controlling with the suitable phase the operations of linear detector DB, of multiplexer 2 and of RAM memory 10. This makes it possible, for example, to reinitialize the potentials at the level of each sensor C1 to Cn after each acquisition of the measuring signal provided by the latter, and to address the memory cells (not shown) contained in RAM memory 10 and which are each intended to store the data which corresponds to the data delivered by that of sensors C1 to Cn to which it is indexed. In short, a memory cell of RAM memory 10 corresponds to each elementary sensor C1 to Cn, and each of these memory cells contains the combination of N values obtained for elementary sensor C1 to Cn being considered.
During the acquisition cycle of signals S1 to Sn or the reading cycle, any new value is added to the contents already stored in the corresponding memory cell; this result is then multiplied by a number N, then divided by N+1, then written in the memory cell in place of the preceding number. In this manner, at any moment, each memory cell of RAM memory 10 contains the weight sum of a large number of samples. The weighting coefficient decreases for each contribution with its seniority. Mathematically, this system is equivalent to a simple low-pass filter of the first order for each pixel; the equivalent time constant is: N.times.T, where T is the period of the acquisition cycle.
Thus, for example, at a given moment when a measuring signal, signal S1, for example, delivered by first elementary sensor C1, is transmitted to analog-to-digital converter 3: the corresponding digital value is applied to a first input ES1 of subtractor 5 as well as to a first input EA1 of an adder 6. At the same time, address counter 8 addresses RAM memory 10 so that the value contained in the memory cell assigned to first elementary sensor C1 is applied to a second input ES2 of subtractor 5 and to a second input EA2 of adder 6. This adder 6 adds the value contained in the memory cell and the new value corresponding to last signal S1; this sum, in the form of digital data, is applied by a connection 16 (of bus type) to a multiplier circuit 11 which performs on this sum the operation N/N+1, where N is the number of acquisitions already performed of signal S1.
The new value defined by multiplier 11 is applied to read-write RAM memory 10, by a supplementary memory 13, to be written in the memory cell concerned in place of the preceding value; buffer memory 13 being necessary to wait for RAM memory 10 to pass from the "reading" phase to a "writing" phase.
During this entire first phase, subtractor 5 does not operate, and at the end of this phase, the contents of the memory cells of RAM 10 are set, and they constitute the corrective terms of the dark current. In the next phase, which can be that of the development of corrective terms of gain, the contents of the memory cells of RAM 10 are applied to subtractor 5 in the same order as measuring signals S1 to Sn, so that the corresponding value of dark current is subtracted from each value of signal S1 to Sn; the result of the subtraction being a corrected data item DC.
This part of the operation where the correction is performed is identical with the operation which is performed in the case where the value of the dark current is determined by a true average method. The method, in the case of the true average, is very close: actually, in this case, after digitalization of the signals, a digital memory (similar to RAM 10) addressed by the number of the elementary sensor (or all one-to-one functions of this number) contains the sum of N successive values (N is often a power of 2). This sum is divided by N (this is often obtained by abandonment of low order bits if N is a power of 2) to constitute an average, and this average is subtracted in the measurement to perform the correction.
In the case of a gain correction, the correction is made by dividing the measurement by the corrective term thus obtained, i.e. that the subtractor circuit is replaced by a multiplier circuit.
Of the drawbacks that the method, which was just described in reference to FIG. 1, presents, there is a particularly troublesome one which resides in the fact that it is necessary to use an analog-to-digital converter with very great precision and very high dynamics: thus, for example, it is usually necessary to use at this level analog-to-digital converters of 12,000 points or more (i.e. of at least 14 bits). As a result, such a converter is of a considerable cost able to represent by itself alone about one third of the cost of the unit. Another very unfavorable point comes from the fact that using an analog-to-digital converter with very high dynamics tends to reduce in a significant way the speed of operation of the unit and leads as a result to limiting the speed of passage of the conveyor belt on which the objects are in movement or to limiting the number of acquisition cycles to develop the corrective terms.